1. Field of the Invention
The present invention relates to a coding device capable of coding a signal by dividing the signal into temporally continuous frames and blocks, and a decoding device for decoding a code sequence generated by the coding device.
2. Description of the Related Art
There exist numerous kinds of methods for efficiently compressing audio signals and coding the signals, and one widely used method involves using a variable bit rate in the coding process. For example, variable bit rate coding is used in AMR (Adaptive Multi-Rate) coding, which is a standard coding scheme in 3GPP (third Generation Partnership Project), a project aiming at standardization of third generation technologies related to cellular phones. In addition, variable bit rate coding is used in AMR-WB (Adaptive Multi-Rate Wide Band) coding, which is also a standard coding scheme in 3GPP for coding wideband speech signals established as G.722.2 by ITU-T, the Telecommunication Standardization Sector for standardization of technologies in telecommunication in the ITU (International Telecommunication Union). Furthermore, variable bit rate coding is used in EVRC (Enhanced Variable Rate Code), a standard of EIA (Electronic Industries Alliance) and TIA (Telecommunication Industries Alliance).
In these coding schemes, the coding bit rate is varied block by block according to the required communication quality and the condition of the communications network. A block is a division of the input data, and has a predetermined length.
When it is necessary to code a frame having a predetermined length at a bit rate not higher than a specified bit rate, an encoder working at the specified bit rate is used. Alternatively, an encoder capable of working at variable bit rates may also be used at the specified bit rate or lower.
However, if taking into consideration human perception characteristics, it is known that among the data of one frame, some data are important for perception but some are unimportant. Therefore, compared with coding all of the data in a frame at the specified bit rate, it is advantageous to make the encoder work at higher bit rates to code the important data to ensure quality of the data, and at lower bit rates to code the unimportant data without caring about the data quality too much, while ensuring the average bit rate over the frame is not higher than the specified bit rate. In this way, data quality is improvable when taking into consideration human perception characteristics.
For example, Japanese Patent Application Laid Open, No. 9-70041 discloses a coding device capable of coding at variable bit rates, in which the bit rate is specified in each specified time interval of the input data, in other words, the bit rate is specified in each block having a predetermined length, while ensuring that the input data having a predetermined length are coded at an average bit rate not higher than a specified bit rate.
In the meantime, in MP3 (MPEG-1 Layer 3) or MPEG-2 AAC (Moving Picture Coding Expert Group 2 Advanced Audio Coding), which are international standard coding schemes in ISO/IEC and widely used in coding audio signals, the bit rate can be more adaptively varied block by block.
In addition, in time-frequency transformation coding used in coding audio signals, by making the block length variable, coding in units of blocks having variable lengths becomes possible. In the time-frequency transformation coding, when frequency characteristics of the input signal vary slowly, the block length is set long and coding is performed after transformation in the frequency domain. When frequency characteristics of the input signal change quickly, the block length is set short and coding is performed after transformation in the frequency domain. By doing this, data distortion can be suppressed, and the coding efficiency can be improved.
Although being capable of variable bit rate coding, the coding device disclosed in Japanese Patent Application Laid Open, No. 9-70041, is a device for coding digital image data, that is, the device performs coding of image data in a temporally discrete manner, while setting a variable bit rate for each unit time period.
On the other hand, in coding audio data, generally, sampled digital audio data in a certain time period are defined as a block of a predetermined length, and coding of the audio data is performed continuously along the time axis. Accordingly, from the view of improving the coding efficiency and the coding quality, the coding device disclosed in Japanese Patent Application Laid Open, No. 9-70041 cannot be applied to coding of digital signals continuously and dynamically distributed in time, for example, the audio signals.